The invention relates to an incandescent lamp, equipped with
a base for connection into a lamp socket, the base comprising a pair of base terminals for receiving an AC supply voltage with frequency f, PA1 voltage conversion means comprising PA1 low voltage incandescent burner means coupled to the secondary winding, PA1 envelope means, comprising at least a first translucent part, the envelope means being fastened to the base and together with the base enclosing the voltage conversion means and the low voltage incandescent burner means.
rectifying means having input terminals coupled to the pair of base terminals and output terminals, PA2 capacitive means coupled to the output terminals and equipped with a series arrangement comprising two capacitors, PA2 switching means coupled to the series arrangement comprising two capacitors for generating a first high frequency voltage out of the voltage present over the series arrangement comprising two capacitors, PA2 transformer means, coupled to the switching means and comprising a primary winding and a secondary winding, for transforming the first high frequency voltage into a second high frequency voltage,
The invention also relates to voltage conversion means for use in such an incandescent lamp and to an adapter for supplying an incandescent lamp from an AC supply voltage source with a high frequency voltage comprising such voltage conversion means.
An incandescent lamp as mentioned in the opening paragraph is known from U.S. Pat. No. 4,998,044. The known incandescent lamp is an incandescent halogen lamp. In the known lamp, the switching means comprises a series arrangement of two switching elements shunting the series arrangement of the two capacitors. The two capacitors and the two switching elements together form a half bridge circuit. The primary winding of the transformer means is coupled between a common terminal of the two capacitors and a common terminal of the two switching elements. During lamp operation the first high frequency voltage is transformed by the transformer means into the second high frequency voltage that is present over the low voltage incandescent burner. The second high frequency voltage has a maximal amplitude that matches the maximal operating voltage of the low voltage incandescent burner means. The capacitances of the two capacitors are equal and are chosen relatively low. Because of these relatively low values of the capacitances, the capacitors are relatively small which makes it relatively easy to integrate the voltage conversion means into the incandescent halogen lamp. Furthermore these low values of the capacitances cause a relatively low mains current distortion which corresponds to a relatively high value of the power factor of the voltage conversion means and to a relatively low value of the total harmonic distortion. At the same time, however, these relatively low values of the capacitances causes the voltage present over the series arrangement of the two capacitors to drop to a very low value two times in every period of the AC supply voltage. In the voltage conversion means comprised in the known incandescent halogen lamp the bridge circuit is a self-oscillating circuit wherein control signals or rendering the switching elements conducting and non-conducting are derived from the current flowing through the primary winding of the transformer means by means of saturable current transformers. With a frequency 2f, however, the voltage over the capacitors becomes so low that the control signals become too weak to control the conductive state of the switching elements resulting in the bridge circuit stopping its oscillation. To be able to start oscillation once more when the voltage over the capacitors has once more reached a value that is high enough, the voltage conversion means of the known incandescent halogen lamp is equipped with a circuit part for restarting the oscillation. This circuit part comprises ohmic resistors, a startcapacitor and a DIAC. The startcapacitor is charged from the voltage over the two capacitors. When the voltage over the startcapacitor is high enough, the DIAC becomes conductive, at the same time renders one of the switching elements conductive and thereby restarts the oscillation of the bridge circuit.
Although the voltage conversion means comprised in the known incandescent halogen lamp allow the realization of a relatively high power factor and a relatively low amount of total harmonic distortion, its use is also associated with some serious disadvantages. The voltage conversion means, since they are integrated in the lamp, are at a relatively high temperature during stationary operation. A DIAC on the other hand generally has a relatively low maximum operating temperature. To make sure that the DIAC still operates under worst case conditions, the voltage conversion means have to be designed so that the amount of power that can be consumed by the low voltage burner is relatively low. Furthermore the favourable effect that the low values of the capacitance of the capacitors comprised in the capacitive means have on the power factor is counteracted to a certain extent by the fact that the supply current drops to zero during a certain time lapse two times in every period of the AC supply voltage. Additionally in a self-oscillating circuit each switching element is rendered conductive while a voltage is present over it. This is called "hard switching" as opposed to "soft switching" meaning that each switch is rendered conductive while the voltage over it is approximately zero. Because of the hard switching a relatively high amount of power is dissipated in the switching elements increasing the total amount of heat generated and thereby the operational temperature of the voltage conversion means. Another effect of the hard switching is the generation of EMI, necessitating the incorporation of a relatively big filter in the voltage conversion means in order to make the lamp meet the requirements regarding EMI. This relatively big filter makes it very difficult to incorporate the voltage conversion means in the lamp. In a self-oscillating circuit, EMI can also be caused by a lack of symmetry of the first high frequency voltage caused by component tolerances.